drm/i915: tune reset dmesg output a bit

We don't want any ERROR for simulated gpu hangs, otoh printing the
error code when the reset failed for real should be interesting.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=71333


lu hua <huax.lu@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>